Researchers can use microwaves to learn the structure of soil without disturbing it, according to the 28 October print issue of PRL. In the new technique, ground-penetrating radar captures the soil’s distribution of solid particles and pores, up to depths of 10 meters. The technique could give agriculturists and environmental scientists a fast and simple way to measure soil properties, which can determine the land’s suitability for crops, say the researchers.
Over the last few decades, scientists have discovered that a host of objects in the natural world, from snowflakes to coastlines, are fractals–objects that replicate the same pattern on many different size scales. A team led by Klaudia Oleschko of the National Autonomous University of Mexico in Mexico City has demonstrated that the arrangement of pores in soil displays fractal geometry on scales from 0.008 millimeters to 1 meter. Now the team has used microwaves to measure soil’s fractal dimension, which indicates the amount of empty space contained in a volume of dirt.
The fractal dimension is a good indicator of a soil’s density, water content, and porosity. Measuring these attributes is key to predicting how well a soil will perform agriculturally, Oleschko says. “Plant roots develop well only in soils that have optimal mechanical and physical properties,” she says. “Agricultural productivity depends directly on these properties.”
When microwaves are beamed at soil, the graph of the reflected waves is itself a fractal. Oleschko’s team showed mathematically and by computer simulation that this fractal has the same dimension as the soil that reflected it. The researchers then used radar to explore the structure of a 1 × 1 × 1.6 meter block of soil consisting of six layers with different soil densities and structures. The radar results gave the fractal dimension for each soil layer and agreed with direct measurements–using optical and electron microscope images–of samples taken from the block.
The team’s technique is much faster than current methods for measuring a soil’s mechanics, Oleschko says. “We were able to map a 1000-square-meter agricultural field in just about 30 minutes, to get measurements that would normally take months of expensive lab work,” she says. What’s more, the technique can be performed without removing the soil from its environment. “Soil is a living, dynamic system, so it is important to be able to make non-invasive measurements,” Oleschko says. “Taking samples to a lab is problematic, because the moment you take the samples, all the soil properties change drastically.”
The use of radar to measure soil’s fractal dimension is a nice extension of the classical idea of measuring a jagged shoreline by using yardsticks of many different sizes, observes Dwight Jaggard of the University of Pennsylvania in Philadelphia. In this case, the different microwave wavelengths function as the yardsticks. “It’s a wonderful use of waves to investigate the characteristic fingerprints that fractal soils leave,” he says.
Erica Klarreich is a freelance science writer in Berkeley, CA.